Func and Action Delegates

The Func and Action generic delegates were introduced in the .NET framework version 3.5. They provide flexible delegates with generic parameters that can be used for many purposes, including passing lambda expressions to method parameters.

Generic Delegates

The .NET framework version 3.5 introduced two new sets of generic, parameterised delegates named Func and Action. The Func delegate can be used to encapsulate a method that accepts between zero and four arguments and returns a value. The Action delegate also represents methods with zero to four parameters but differs from Func in that the method must return void.

The new delegates can be used to reduce the number of delegates that you define explicitly. In situations where you would need a delegate that matches one of the predefined Func or Action signatures, you may decide to use the built-in version. You should consider the naming of the delegate however, as "Func" or "Action" may not express your intent as clearly as another name.

One of the key reasons for the introduction of Func and Action is their relationship with lambda expressions. Every lambda expression's underlying type is one of these generic delegates. This means that lambda expressions can be passed to method parameters of the appropriate type without explicitly creating a delegate. Many of the LINQ standard query operators accept Func arguments to take advantage of this feature.

Func Delegate

There are five variations upon the Func delegate, each allowing a different number of parameters to be represented. In each case, the return value and the parameters are defined as generic types, allowing the types to vary according to the method that the delegate encapsulates. The five signatures are shown below. Note that the return value of the method is always the last element in the list of types.

Using Func<TResult>

The most basic variant of Func is Func<TResult>. This delegate represents methods that return a value but have no parameters. We can demonstrate this with the following sample code. In the example, a new delegate is created using an anonymous method. The method returns a double-precision floating-point number containing a tax rate value.

Again, the code can be recreated using a lambda expression in place of the anonymous method. In this case, the three parameters are named a, b and c. The parameter types do not need to be included in the lambda expression as they, and the return type, are inferred by the compiler.

Action Delegate

As with Func, the Action delegate has five variations. These allow the encapsulation of methods that have up to four parameters but do not return a value. Again, all of the parameters are generic types allowing any type to be used for each argument. The five signatures are as follows:

Using Action with Parameters

As a final example of the basic use of Action, consider the following example. This code uses a single string parameter for the delegate. The parameter accepts a message to be outputted to the console.